They own the facility, which means that they are fully in control of the scheduling, they do not need to lease the launchpad and support facilities at Cape Cana

They own the facility, which means that they are fully in control of the scheduling, they do not need to lease the launchpad and support facilities at Cape Canaveral, and they also get a slight boost from launching closer to the equator. They literally picked the southernmost spot as they could in the continental United States. Lastly, given that Boca Chica was little more than a tiny retiree town, placing the facility there means they can buy up and develop all the land around it far more easily than picking some other coastal town.